This invention is directed to an improved process for spraying molten material in order to recover the material as solidified grains. The process employs a symetrical-rotating reservoir which rotates about a vertical axis and whose upright walls are provided with apertures for spraying molten material. The reservoir is designed so as to accommodate a member provided with blades and rotating at a speed different from that of the reservoir. The process and device is suitable for producing such round granular material as used for example in fertilizer products.
A suitable device known in the art for preparing such round granules is described in U.S. Pat. No. 3,055,049 (the contents of which are incorporated herein by reference). According to this prior art device, molten material is fed to the rotating reservoir and sprayed through spray apertures located in the upright wall of the reservoir. The resulting molten material cools when falling a certain distance, and solidifies to form round granules. These round solid granules are conventionally referred to as "prills". The device employed is commonly referred to as a "prilling bucket" or "prilling head", and generally is mounted at the top of a cylindrical tower. As described in the above-mentioned U.S. Patent, the member provided with blades functions as a scraper and maintains the inside of the prilling head free of crusts of molten material while in use. The distance between the edges of the scraper blades and the inner wall of the prilling head may at most be a few millimeters.
Known processes for preparing prills are particularly well adapted for materials which in the molten state maintain a reasonably constant and not too high viscosity such as for example, urea. However, if the molten material to be granulated possesses a high and sometimes variable viscosity such known processes are not entirely satisfactory. These problems occur particularly with the so-called NPK-fertilizers. The unsatisfactory operation of such prior processes is primarily due to the fact that the static pressure difference between the inner surface layer of molten material on the inner wall of the reservoir and the outer wall of the reservoir caused by the centrifugal force in the prilling head, is insufficient to force an adequate amount of the viscous melt through the spray apertures.
Accordingly, it is the primary object of the present invention to provide a process by which it is possible to satisfactorily process material having high and possibly variable viscosity to form prills. In principle, one might suggest feeding the molten material to the reservoir at a elevated pressure. However, this would require the use of a sealing which would be difficult to employ due to temperature and corrosion problems. Also problems arising from the formation of crusts and blocking by solidifying material when the installation is not in use, preclude such sealing means.